In order to obtain a high degree of immersion at high towing speeds, the towing cable is faired to reduce its hydrodynamic drag to a very large extent. FIG. 1A depicts a portion of the cable 1 extending along an axis x. This cable is faired, namely covered with fairing elements having shapes intended to reduce the hydrodynamic drag. The fairing elements form a fairing also referred to as a fairing string. The fairing elements are rigid. In other words, they do not deform under the effect of the hydrodynamic flow. The cable 1 is conventionally faired by means of a fairing or fairing string 3 comprising a series of fairing elements 2 or fairings. Each fairing element 2 comprises an elongate element exhibiting a hydrodynamic profile. The hydrodynamic profile is the shape of a cross section of the fairing portion in a plane perpendicular to the axis x. The hydrodynamic profile of the fairing elements is, for example, as depicted in FIG. 1B, in the shape of a wing having a thick internal edge (or leading edge BA) housing a tubular canal through which the cable 1 passes and a thin external edge (or trailing edge BF) allowing a less-turbulent flow of the water around the cable. The hydrodynamic profile exhibits, for example, a teardrop shape or is an NACA profile which is a profile defined by the National Advisory Committee for Aeronautics, NACA. The collection of fairing elements completely or partially covers the cable. The fairing elements are immobilized translationally with respect to the cable along the axis x.
In the normal operating state, the fairing elements are mounted with the ability to rotate about the cable, namely about the axis x. However, each fairing element is connected to its two neighbors in such a way as to be able to pivot with respect to these about an axis parallel to the axis x by a maximum angle that is small, of the order of a few degrees. This is because it is necessary for the fairing elements to be able to rotate freely about the cable so as to be correctly orientated in the various phases because it is not possible to control the orientation of the cable itself; these phases are: orientation according to the stream of the water, orientation in order to pass through the pulleys, reeling, of the guide device and storage on the drum. As a result, the rotation of one scale leads to a rotation of the neighboring fairing elements and so on and so forth through the entire set of fairing elements. Thus, both when the cable is deployed in the water and when it is wound around the drum, any change in orientation of one of the fairing elements has a knock-on effect on all of the fairing elements fairing the cable. Thus, when the cable is deployed at sea, the fairing elements naturally oriented themselves in the direction of the current generated by the movement of the vessel. Likewise, as the cable is wound around the drum of the winch, all the fairing elements, as the cable is raised, adopt one and the same orientation relative to the drum, which orientation allows the cable to be wound in keeping the fairing elements parallel to one another turn by turn.
Now, the applicant company has found that, when the cable is wound around the drum so as to recover the towed body, the fairing sometimes becomes severely damaged or even crushed as it passes through the guide devices, this being something which may render the entire sonar system unavailable. It may even happen that this damages the guide device. By way of example, certain variable-immersion sonar systems installed on certain ships and operated in the normal way by military crews encounter fairing-element-crushing problems approximately once a year and sometimes far more frequently. This situation causes the system to become unavailable for a period which may range from a few hours to a few months, during which maintenance operations have to be carried out.